The present invention relates in general to electronic speed controls for alternating current motors, and more particularly to a transistor circuit for varying the amount of line voltage applied to a multipole induction motor driving a constant load, such as a relatively low rpm ceiling fan of the "Casablanca" type. The induction motor's speed will change due to more or less pole slippage caused by varying the applied power.
It is known in the art that the voltage, and thus the power, applied to an alternating current load can be varied by using adjustable transformers such as Variacs or similar autotransformers. While such transformers may provide suitable power regulation, as passive devices they must necessarily dissipate heat generated in part by unused wattage not applied to the motor. Also, where the load is inductive, reactive current flowing through the transformer adds to the amount of heat that must be dissipated. Further, such transformers are bulky and generally high in cost due to the large amount of copper wire needed to handle relatively high currents.
One solution to the problems noted above lies in the use of well known phase control thyristor circuits that regulate the amount of power applied to an alternating current load by varying the time of voltage application to the load on a half-cycle basis. Such switching circuits, while having known advantages over variable transformers, may cause noise generating current spikes and other undesirable transients, since the sinusoidal wave shape of the applied voltage is abruptly reshaped by the thyristor switching. Where the load is an induction motor, clogging of its armature and movement of its laminations and windings may also occur because of the non-sinusoidal characteristics of the applied voltage.
It is therefore desirable to maintain the sinusoidal characteristics of the applied voltage while reducing or increasing its amplitude as a function of time to reduce or increase the amount of power applied to the motor.
This feature can be provided by using a single transistor for controlling both forward and reverse current pulses applied to the motor, appropriate bridge-configured diodes routing the current pulses through the transistor. Such a single transistor-type motor speed control requires that the transistor dissipate as much as 35 watts of power in the form of waste heat. The temperature of the junction of the transistor can easily reach unacceptable levels, resulting in erratic operation and shortened transistor life.